Most likely, 1) you're looking in the wrong place, 2) the sync process hasn't completed, or 3) you've received a sync error on one of your computers.
In Zotero libraries, all existing items are shown in either the library root or the trash. Make sure you're not looking in a collection that contains only some of your items. If you don't see a group in the Zotero client, make sure “Group Libraries” is expanded in the left-hand pane.
If you're sure you're looking in the right place, check to see whether the data in question has synced to your online library on zotero.org. If it hasn't, the problem is on the computer where the data originated. If it has, the problem is on the computer missing the data.
Check the green sync icon in the Zotero pane on the relevant computer. If it's still spinning, the sync process hasn't yet completed. Hover over the sync icon to see the current status. If it has been spinning for a while, see Sync Queuing.
If you see a red error icon to the left of the sync icon, an error has occurred during the sync. Click the icon to view the error. It may give you enough information to fix the problem yourself, or you can report it and post the Report ID to the forums for further assistance.
Also check the Sync pane of the Zotero preferences to make sure you're using the same account you're using to log in to zotero.org and the same account you're using on the other computers (or, in the case of a group, the account that's a member of the group).